At least you had the coal. All I managed to grab was 'fools coal'.
I researched steam power and didn't have any coal in my border. But there was a source just across my border with England. So I built a city right beside it and moved the border.
But the English had moved a worker there just before this and started clearing the jungle.
I waited the 24 turns, then sent my own worker there to build a road. The very instant the road was built, I got the message saying the coal was exhausted!
By what? No one could have used it until I got the road there!
Obviously, a vein of fool's coal.
Gotta love this game. I have to keep thinking around it.